Inductor and method for producing same

ABSTRACT

There is provided a gap in a magnetic circuit of an iron core inductor, and a plurality of small size permanent magnet pieces are placed in the aforesaid gap, with the magnetizing directions thereof being provided in side-by-side relation. If the magnetizing direction of the permanent magnet pieces is opposed to that of a D.C. magnetic field which is produced in a magnetic circuit, yet in case D.C. and A.C. overlapping currents flow through the inductor, then the D.C. magnetic field produced in the magnetic circuit will be off-set thereby, providing a high inductance value, while eddy currents produced within the permanent magnet pieces due to the A.C. magnetic field will be reduced to a considerable amount, because the permanent magnet pieces are small in size, thus minimizing the heat to be produced in the permanent magnet pieces.

BACKGROUND OF THE INVENTION

This invention relates to an inductor consisting of such an iron coremade of a soft magnetic material, which serves as a magnetic circuit,around which is wound a coil, and more particularly to an inductorhaving a permanent magnet in part of the magnetic circuit thereof.

With an inductor for use in an electric circuit, through whichoverlapping A.C. and D.C. currents flow, as in the case with a chokecoil, the iron core of the inductor is brought to a magnetic saturationdue to D.C. electric current. For this reason, extremely lowpermeability of the iron core will result, and thus effective inductanceof an inductor will be lowered.

To avoid this shortcoming experienced with the prior art inductor foruse in an electric circuit, through which a great amount of D.C. currentflows, there has been proposed a method, whereby to avoid the magneticsaturation in the iron core by providing a gap in part of the magneticcircuit thereof. However, the provision of a gap in part of the magneticcircuit will lead to the lowered permeability of the entire iron corewhich in turn results in lowered inductance of an inductor.

Another attempt to avoid the magnetic saturation in the iron coreaccruing from the D.C. current, which flows through an inductor, is thatthere is inserted in part of the magnetic circuit a permanent magnethaving a magnetic field of an intensity of the same level as that of theD.C. magnetic field, with the direction of the magnetic field thereofbeing opposed to that of the D.C. magnetic field induced within the ironcore due to a D.C. current. With the inductor of the aforesaidarrangement, the D.C. magnetic field induced within the iron core due tothe D.C. current is off-set by the magnetic field produced by apermanent magnet, whereby there will not arise the magnetic saturationin the iron core made of a soft magnetic material.

This permits the iron core to operate at an extremely high level ofpermeability for an A.C. current, thereby providing a high inductance.

On the other hand, since not only the D.C. magnetic field an A.C.magnetic field are applied to the iron core of an inductor at the sametime, materials having as high a specific resistance as possible areused as an iron core of a soft magnetic material, for instance, softferrites such as Mn-Zn ferrite and Ni-Zn ferrite and the like. Thespecific resistances of those materials are no less than 10² Ωm. For thesame reason, a hard ferrite magnet having a specific resistance of noless than 10² Ωm, such as for instance, a Ba-ferrite magnet is used aspermanent magnet to be inserted in part of the iron core of a softmagnetic material, of the inductor.

The coercive force of the hard ferrite magnet ranges from 2000 to 4000oersted which is greater as compared with that an Alnico magnet, suchthat there will not be caused demagnetization by the use of the hardferrite magnet for a portion, where A.C. magnetic field of a high levelas well as a D.C. magnetic field having an opposed magnetizing directionare simultaneously applied as in the case with an iron core of aninductor.

On the other hand, the residual flux density of the hard ferrite magnet,as well known, is 3000 to 4000 gauss. Thus, the residual flux densitythereof is half or one third as much as that of a metal magnet such asAlnico magnet. Accordingly, the use of the hard ferrite magnet as abiasing magnetic field source of an inductor is limited to the casewhere the D.C. component of the electric current is relatively small inamount. On the other hand, in case a D.C. component is relatively great,the D.C. magnetic field induced by the D.C. current will be much greaterthan that produced by the hard ferrite magnet, with the result thatinductance of an inductor for the A.C. current will be lowered. Forproviding a higher level of biasing magnetic field by using an inductorwhich incorporates a hard ferrite magnet as a biasing magnetic field, itis required to use a magnet having a larger magnetic pole area. The useof a magnet having a larger magnetic pole area then dictates the use ofa magnet of an increased length for avoiding demagnetization due to thedemagnetizing field. (The thickness of the inserted magnet with respectto a direction of the magnetic circuit should be increased.) As has beendescribed, with an inductor using as a biasing magnetic field a hardferrite magnet which is inserted in part of a magnetic field, the sizeof the inductor should be increased, when an electric current having agreat amount of D.C. current component is to flow therethrough.

SUMMARY OF THE INVENTION

It is accordingly a principal object of the present invention to providean inductor which is provided with a magnetic field, in part of which isinserted a metallic permanent magnet as a biasing magnetic field.

It is another object of the present invention to provide an inductorwhich minimizes the heat produced within a metallic permanent magnetused as a biasing magnetic field, even for a A.C. current having a highfrequency.

It is a further object of the invention to provide an inductor whichuses as a biasing magnetic field a metallic permanent magnet having anexcellent or desired magnetic characteristic, residual flux density,coercive force and the like, such as for instance, platinum-cobaltmagnet or rare earth-cobalt magnet.

It is a further object of the invention to provide an inductor whichprovides an inductance of a sufficient level, even in case the D.C.current of an extremely great amount flows through an electric circuit,through which overlapping A.C. and D.C. currents flow.

It is a yet further object of the invention to provide an inductor whichprevents demagnetization of a permanent magnet used as a biasingmagnetic field, even when an impulse current such as a surge currentflows through an inductor and which prevents no variation in inductance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an inductor of a construction, wherein abiasing permanent magnet is inserted in a gap defined in part of amagnetic circuit;

FIG. 2 is a plan view of a plurality of permanent magnet pieces arrangedin a flat plane, which magnets are adapted for use in an inductoraccording to the present invention;

FIG. 2A is a side elevational view in section of a further embodiment ofthe arrangement of the plurality of magnet pieces disposed in the gap ofFIG. 1;

FIG. 2B is a side elevational view in section of still a furtherembodiment of the arrangement of the plurality of magnet pieces in a gapsuch as FIG. 1;

FIG. 3 is an explanatory view for use in calculating eddy current lossin a permanent magnet plate;

FIG. 4 is a cross-sectional view of one embodiment of a permanent magnetused in an inductor according to the present invention;

FIG. 5 is a plan view of another embodiment of a permanent magnet foruse in an inductor according to the present invention;

FIGS. 6, 7 and 9 are front views of embodiments of an inductor accordingto the present invention, respectively;

FIG. 10 is a side view of a further embodiment of an inductor accordingto the present invention;

FIG. 8 is a wedge-shaped adjusting piece for use in an inductor of FIG.7;

FIG. 11 shows magnetic characteristics of an iron core used inembodiments of the present invention;

FIG. 12 is a plot showing the relationship of inductance of anembodiment of an inductor according to the present invention and that ofan inductor of the prior art versus D.C. overlapping currents; and

FIG. 13 is a plot illustrating the relationship between the inductanceof an inductor of another embodiment of the present invention and D.C.overlapping currents.

DETAILED DESCRIPTION OF THE INVENTION

These features of the inductor according to the present invention willbe described in more detail hereunder, in which a plurality of discretepermanent magnet pieces are located in a gap provided in an iron coremade of a soft magnetic material and forming a magnetic circuit, aroundwhich is wound a coil.

FIG. 1 shows a front view of one embodiment of an inductor. Shown at 11,11' are an E-type iron core made of a soft magnetic material, at 12 acoil which is wound around the central leg 13 and outer legs 14 and 15.The E-type iron core 11' has a central leg 13' and outer legs 14', 15'.The central legs 13, 13' are shorter than the outer legs 14, 15, 14',15', respectively, such that when the E-type iron cores 11 and 11' areassembled together in abutting relation to each other, then there willbe provided a gap between the opposing end faces of the central legs 13and 13'. A plate-form permanent magnet 16 is inserted in this gap. D.C.and A.C. overlapping currents flow through a coil 12 wound aroundcentral legs 13 and 13', continuously. It is designed that a D.C.magnetic field produced within the central legs 13 and 13' and theintensity of magnetization of the permanent magnet 16 have the oppositedirection and provide substantially the same level. According to thepresent invention, the plate-form permanent magnet 16 is divided into aplurality of small-size magnet pieces 21, as shown in FIG. 2 (25 innumber in the drawing), with the magnetizing directions thereof beingprovided in side-by-side relation.

The heat to be produced within the permanent magnet 16 due to an A.C.current flowing through the coil of an inductor will be expressed asfollows:

Assume that the permanent magnet 16, as shown in FIG. 3, is of a platehaving an area of S m², and of a square shape having one side of alength of √S m and a thickness of a m. Further assume that an A.C.current flowing through the coil produces uniform A.C. magnetic fieldrunning through the permanent magnet 16 in a direction perpendicular tothe surface of the plate 16. The A.C. magnetic field is expressed by thefollowing formula:

    B =  B.sub.m cos 2 π ft

, wherein B_(m) represents the instantaneous maximum value (Wb/m²) ofmagnet flux density, f does a frequency (Hz), and t does a time (sec).On the other hand, the unit of A.C. magnetic field is given in Wb/m².

The effective value of an electromotive force induced at the minuteportion 3 having a minute width dx, in the permanent magnet 16, isexpressed as follows:

    E (volt) = √2 πf B.sub.m (2x).sup.2

Assume of specific resistance of permanent magnet 16 of ρΩm, then thecircuit resistance R of the minute portion 3 will be expressed asfollows: ##EQU1## Accordingly, the power loss dp produced at the minuteportion 3 will be such as: ##EQU2##

The power loss P produced in the permanent magnet in its entirety willbe as follows: ##EQU3## The above formula may be expressed as follows:

    P = K .sup.. S.sup.2,

wherein ##EQU4##

As is apparent from the foregoing, the power loss produced in thebiasing permanent magnet 16 is in proportion to the square of the crosssectional area S. As is the case with the present invention, in case apermanent magnet is divided into sections of `N` in number, then therespective areas of the discrete magnet pieces will be S/N m² . Thus,the power loss in the respective magnet pieces, i.e., the eddy currentloss Δ P = k. (S/N)².

The total power loss of the discrete magnet pieces will be N. Δ P. Thisvalue is k. S² /N which is reduced to 1/N, as compared with the loss P=k. S² prior to division of the magnet.

As is clear from the foregoing, the inductor according to the presentinvention which uses discrete permanent magnet pieces as a biasingpermanent magnet permits to minimize the eddy current loss within thebiasing permanent magnet due to such an A.C. magnetic field within aniron core, which is induced by the coil, thereby preventing the heatproduced in the permanent magnet.

Even in case a metallic magnet having a high electric conductivity, suchas case Alnico magnet, sintered Alnico magnet, rare earth cobalt magnet,etc., is used as a biasing permanent magnet, the magnet divided intosmall discrete magnets and incorporated in the magnetic circuit willminimize the eddy current loss caused due to an A.C. magnetic field andhence the heat in the magnet.

Since those metallic magnets have excellent magnetic characteristics andproduce a high biasing magnetic field in an iron core of an inductor,they can off-set D.C. magnetic field produced by a great amount of D.C.current flowing through the coil in overlapping manner, thus aiding inthe conductor exhibiting excellent characteristics.

Now, description will be given on the method for producing an inductoraccording to the present invention. For the production of the inductor,the magnetizing directions of a plurality of discrete magnet piecesshould be arranged in side-by-side relation, before a plurality ofdivided permanent magnet pieces are inserted in the gap defined betweenthe iron cross of the inductor. However, difficulties arise in arrangingthe magnetizing directions of discrete magnet pieces, because there willarise repulsion between the discrete magnet pieces having the samepolarity, due to the proximity of those magnet pieces, with the resultthat those magnet pieces cannot be set within a given area or are apt tobe superposed one on top of another, thus failing to be arranged inorder without interstice between adjacent magnetic pieces.

One embodiment of the method for producing an inductor according to thepresent invention will be described with reference to an inductorprovided with a E-type iron core having a magnetic circuit as shown inFIG. 1, wherein there is provided a gap between the end faces of thecentral legs 13 and 13'. An adhesive is applied to the end face of thecentral leg 13' of the E-type iron core 11', and then a plurality ofpermanent magnets, such as small size magnet pieces 21 as shown in FIG.2 are placed in side-by-side relation on the end face thus prepared.Then, the E-type iron core 11 on one side is assembled to the iron core11', with the permanent magnet pieces thus placed being interposedbetween the end face of the central leg 13 of the iron core 11 and theend face of the central leg 13'. Finally, a coil is wound therearound toprovide an inductor. In this manner, the respective permanent magnetpieces may be fixedly attached in the gap defined between the legs 13and 13' by using an adhesion force of the adhesive which overcomes arepulsion force of the respective magnetic poles, thus facilitatingassembly of an inductor. The adhesive as used herein should preferablybe of an epoxy type having a high viscosity, because the adhesive ofhigh viscosity permits ready assembly, even immediately beforesolidification of the adhesive. Furthermore, the step of placing therespective permanent magnet pieces on the layer of the adhesive permitsthe adhesive to make ingress into interstices among the respectivepermanent magnet pieces, thus preventing the flowing of an eddy currentproduced within the respective permanent magnet pieces into otherpermanent magnet pieces, thus further minimizing the eddy current loss.

Another embodiment of the method for producing an inductor according tothe present invention is as illustrated in FIG. 2A. An adhesive isapplied on the surface of a sheet 16A having a electrically insulatingproperty, and then a plurality of permanent magnet pieces 21 are placedin order, to thereby be bonded to the surface of the sheet in anintegral fashion, after which the sheet 16A thus prepared is inserted ina gap such as that illustrated in FIG. 1 provided in the magneticcircuit, and then a coil is wound around the magnetic circuit to providean inductor. This method permits ready assembly, with the feasibility ofefficient mass production.

A further embodiment of the method for producing the inductor accordingto the present invention is that a plurality of permanent magnet piecesare placed in a flat plane, with the magnetic poles thereof beingarranged in the same direction, then resin is poured thereon in a mold,after which the adhesive attaching to the magnetic pole surfaces of thepermanent magnet pieces is removed. Finally, a plurality of magnetpieces thus prepared are inserted in a gap in a magnetic circuit. Thistype of a method facilitates assembly, while eliminating magneticleakage, because the magnetic pole surfaces of the respective permanentmagnet pieces contact directly the end faces of the leg portions of theiron cores.

A still further embodiment of the method for producing an inductoraccording to the present invention is as illustrated in FIG. 2B.

There is provided a plurality of recessed portions 13A in both or eitherof the end faces of the legs 13, 13' of the iron cores, which end facesare placed in opposing relation to each other, with a gap beinginterposed therebetween. In this respect, the depths of the respectiverecessed portions would be smaller in dimension than the thickness ofthe permanent magnet pieces. Then, the respective permanent magnetpieces 21 are placed in the aforesaid respective recessed portions, withthe magnetic poles being arranged in the same direction. This type ofmethod permits to place the magnets having magnetic poles of the samedirection in a plane, simply by placing the respective permanent magnetpieces in the respective recessed portions, resulting in ready assemblyof iron cores. In addition, since the gap length in the iron cores issmaller in dimension than the thickness of the permanent magnet pieces(i.e., the length of the permanent magnet piece in a direction of amagnetic pole), the magnetic flux will be produced so as to run throughthe gap between the iron cores rather than through the permanent magnetpieces, even if a great amount of D.C. impulse currrent flows throughthe inductor, such that demagnetization of the permanent magnet piecesmay be prevented, thus presenting a consistent inductance.

A yet further embodiment of the method for producing an inductoraccording to the present invention is as follows:

A plurality of permanent magnet pieces are placed on the end face to oneof the legs of iron cores, and then the other magnetic core is placed soas to abut the aforesaid permanent magnet pieces are fixed in positionby means applied from outer circumference of the permanent magnetpieces, followed by magnetization. This type of method permits theorderly arrangement of the permanent magnet pieces on the end faces ofthe legs of the iron cores without considering the directions of themagnetic poles of the respective magnet pieces, thus facilitatingassembly of an inductor.

A further embodiment of the method for producing an inductor accordingto the present invention is that a plurality of grooves are provided inthe top and back surfaces of a permant magnet plate which has beenmagnetized in a direction perpendicular to the surface of the plate, andthen the permanent magnet plate is inserted in a gap defined in themagnetic circuit. Then, a coil is wound around the magnetic circuit tocomplete the assembly of an inductor.

More particularly, as shown in the cross sectional view in FIG. 4, apermanent magnet plate 41 is magnetized in a direction perpendicular tothe plate surfaces 42 and 42', while grooves 43 and 43' . . . . areprovided in the surface 42, and grooves 44, 44' . . . . are provided inthe back surface 42'. In case the permanent magnet plate 41 thusprepared is inserted in a gap defined in the magnetic circuit of aninductor, then there will be induced eddy currents in the aforesaidpermanent magnet plate 41 due to an A.C. component of a current flowingthrough an inductor. The permanent magnet plate 41 presents a functionlike having a high electric resistance partially, because of the smallcross sectional area of the plate 41 where the grooves 43, 43', 44, 44'are present. The eddy current thus produced is interrupted at portionsof the grooves 43, 43', 44, 44', while the eddy current will flow withinthe small regions separated or confined by the grooves formed in thesurface of the permanent magnet plate, because of the increasedresistance at the aforesaid small cross-sectional area portions. Thisthen minimizes the eddy current loss to a great extent, as compared withthe permanent magnet plate free of grooves, with the result of decreasedheat production.

FIG. 5 is a plan view of a further embodiment, wherein there areprovided a group of grooves 53, 53' . . . and another group of grooves54, 54' in the top surface 52 of the permanent magnet plate 51 and inthe back surface 52' thereof, respectively, with the directions of theaforesaid two groups of grooves being provided at a right angle to eachother. This arrangement minimizes the eddy current loss to a furtherextent, as compared with the case shown in FIG. 4.

A still further embodiment of the method for producing an inductoraccording to the present invention, wherein a plurality of permanentmagnet pieces are placed in a gap formed in a magnetic circuit tothereby provide an inductor presenting magnetic biasing, is that thereis provided gap in such a manner that the extent of at least part of theaforesaid gap is smaller in dimension than the thickness of thepermanent magnet piece. More specifically, referring to FIG. 6, thereare provided two `U` shaped iron cores 61, 61', with their legs facingeach other. In this case, there is provided a gap 62 between theopposing end faces of the legs on one side, the extent of part of theaforesaid gap 62 being less in dimension than the thickness of thepermanent magnet 63. The permanent magnet 63 includes a plurality ofpermanent magnet pieces therein, with the direction of magnetizationthereof being opposed to that of the D.C. magnetic field running throughthe magnetic circuit, whereby the permanent magnet 63 will function as abiasing magnetic field for the D.C. magnetic field.

The magnetic flux created by the biasing magnetic field produces asufficient biasing magnetic field, as shown by the solid line, becausethe magnetic flux extends through an almost closed magnetic circuit. Ahigh level of the demagnetizing field, which is created due to flowingof an impulsive D.C. current through a coil of an inductor, runs throughsuch a portion of the gap 62 having a smaller gap extent, as shown bythe broken line having arrow marks, such that the demagnetizing fieldwill not directly applied to the permanent magnet 63, thus preventingdemagnetization of a permanent magnet, with the result that theinductance of the inductor may be stabilized.

A further embodiment of the method for producing an inductor accordingto the present invention is as follows: This is provided a magneticbypass circuit between the iron cores, with a gap being interposed inone portion in the magnetic circuit between the iron cores facing eachother, the aforesaid magnetic bypass circuit being made of a softmagnetic material and adapted to vary the extent of the magnetic bypass,whereby inductance of an inductor may be adjusted as required.

As shown in FIG. 7, there are provided two U-shaped iron cores 71 and71', with the legs thereof facing each other. The legs on one sidedefine a gap 72, in which there is placed a permanent magnet 73consisting of a plurality of permanent magnet pieces. Shown at 75 is anadjusting piece of a wedge shape which is made of a soft magneticmaterial, which is provided on the side surfaces of the iron cores so asto magnetically shortcircuit the iron core legs 76 and 76' which faceeach other through the medium of the permanent magnet 73. The aforesaidwedge shaped adjusting piece 75 functions as a magnetic bypass circuit.The magnetic resistance varies depending on the set position of thewedge-shaped adjusting piece 75. When the wedge-shaped adjusting piece75 is moved to a position shown by the one point broken line as shown inFIG. 7 (A), then the legs 76 and 76' facing each other will bemagnetically shortcircuited. Accordingly, the value of the magnetic biasdue to the permanent magnet will be decreased. On the other hand, whenthe wedge-shaped adjusting piece 75 is moved to a position shown by onepoint broken line in FIG. 7 (B), there will result a reduced extent ofthe magnetic shortcircuiting, while the magnetic flux produced in thepermanent magnet 73 will run in its majority through the iron cores 71and 71', such that the value of the magnetic bias will be greater. FIG.8 shows one example of the wedge-shaped adjusting piece used for theinductor shown in FIG. 7. The dimension shown is given in meter.

With the inductor as shown in FIG. 9, two U-shaped iron cores 91 and 91'are placed, with their legs facing each other, while a permanent magnet92 consisting of a plurality of permanent magnet pieces is placed in agap defined between the legs facing each other. A soft magnetic materialpiece 93 is provided through the medium of a non-magnetic material layer94 on the sides of the iron cores for magnetically shortcircuiting thelegs 95 and 95' of the iron cores, which face each other, with apermanent magnet 92 being interposed therebetween. In this case, themagnitude of the biasing magnetic field may be adjusted by varying thethickness of the non-magnetic layer 94.

With the inductor shown in FIG. 10, there is provided a disk-like softmagnetic material 103 which is adapted to shortcircuit the legs 101 and101' of the iron cores, with the biasing permanent magnet 102 beinginterposed therebetween. The soft magnetic material 103 is adapted torotate about the center of the gap defined by the legs of the ironcores, which face each other. The disk-like soft material 103 is formedwith cut-away portions 104 and 104' which extend from the outercircumference of the material 103 to the center thereof, i.e., the axisof the material 103, whereby the rotation of the disk-like soft magneticmaterial 103 in a direction shown by an arrow will vary inductance of aninductor.

As has been described with reference to FIGS. 7 to 10 thus far, theprovision of the adjustable magnetic bypass circuit for the legs of ironcores, with face each other, permits the adjustment of the magnitude ofthe magnetic bias caused by the permanent magnets, whereby theinductance of the inductor may be adjusted to a desired value, asrequired.

The following examples are illustrative of the features of the presentinvention, wherein numerical explanation is given.

EXAMPLE 1

Iron cores of an inductor were prepared to a shape shown in FIG. 1. Thetotal width of the iron core was 57mm, the height thereof 48mm, thewidth of the central leg of the iron core 19mm, cross section of thecentral leg being of a square shape of 19 × 19mm. The soft magneticmaterial forming the iron cores of the inductor is a soft ferrite, whosemagnetic characteristics are shown in FIG. 11. Two E-type iron cores 11and 11' define a gap therebetween, and the gap length thus defined was0.9mm in dimension.

The permanent magnet used was of a flat plate having the magnetic polesurface of 19 × 19mm and a thickness of 0.9 mm. This permanent magnet isa rare earth - cobalt magnet providing magnetic characteristics, such asresidual magnetic flux density Br=8000 gauss, coercive force Hc=8000oersted and electric specific resistance ρ = 5 × 10⁻ ⁷ Ω.sup.. m. Theaforesaid magnet of a flat plate form was divided along the magneticpole surfaces into 25 discrete magnet pieces 21 having the same size.The aforesaid discrete magnet pieces 21 had been magnetized in awidthwise direction.

Then, 25 permanent magnet pieces 21 were placed in a gap defined betweenthe central legs 13 and 13', with the magnetizing directions thereofbeing arranged in side-by-side relation and then bonded together byusing an epoxy base adhesive. Then, a length of copper sheet having awidth of 25mm and a thickness of 0.3mm was wound 15 turns around thecentral legs 13 and 13' through the medium of an insulating film to givea coil 12.

FIG. 12 (A) shows inductances, when an electric current havingoverlapping D.C. and A.C. components flow through the coil 12 of theinductor thus prepared. FIG. 12 is illustrative of the relationship ofinductance L μ H versus the D.C. overlapping current Idc ampere. FIG.12B shows inductance of the prior art inductor for comparison purpose.In this respect, the prior art inductor has the same size as that of theembodiment of the present invention and is free of a biasing permanentmagnet.

In case an electric current having an overlapping A.C. current of afrequency of 20 KHz and D.C. current of 25 amperes was caused to flowthrough the coil of an inductor according to the present invention, theeddy current loss in the permanent magnet portion was found to be 5.8watt and the temperature rise was 53°C. In contrast thereto, the eddycurrent loss when an integral permanent magnet plate was used in theinductor shown in FIG. 1, was 145 watt, while the temperature rise wasconsiderably high, with the result of extremely low inductance, thusfailing to meet the practical use.

EXAMPLE 2

An inductor as shown in FIG. 7 was prepared by using two U-shaped ironcores made of a soft magnetic material. The material of the iron cores71 and 71' are a soft ferrite having characteristics as shown in FIG.11. The permanent magnet for biasing magnetic field was prepared bydividing into 16 sections which are of a square shape of 2.5 × 2.5 mm,and there were used a rare earth element-cobalt magnet having thecharacteristics, such as area of a magnetic pole of 10 × 10mm, athickness of 0.8mm, residual magnetic flux density Br=6000 gauss, andcoercive force Hc-6000 oersted. Those 16 discrete magnet pieces 73 wereplaced in a gap having an extent of 0.8mm in dimension, with themagnetizing directions thereof being arranged in side-by-side relationand then bonded in position by using an epoxy-base adhesive. A copperwire having a diameter of 0.65 and coated with polyester was woundtherearound 300 turns. An adjusting piece 75 of a wedge shape shown inFIG. 8 was prepared by using the material the same as that of theU-shaped iron cores 71 and 71'. The size of the wedge-shaped adjustingpiece 75 is shown in FIG. 8. When the adjusting piece 75 of a wedgeshape is moved to a position (A) as shown in FIG. 7, the relationshipbetween the D.C. current Idc ampere flowing through the coil 74 and theinductance LmH will be such as is shown in FIG. 13(A). On the otherhand, when the wedge-shaped adjusting piece 75 is moved to a position Bin FIG. 7, the relationship between D.C. current and inductance will besuch as is shown in FIG. 13(B).

What is claimed is:
 1. An inductor device comprising:a magnetic circuitmade of soft magnetic material and end faces defining a gap in oneposition in said circuit; a coil wound around a portion of said magneticcircuit; and a plurality of permanent magnet pieces disposed in said gapbetween said end faces said permanent magnet pieces being substantiallydisposed in a common plane and spaced in said plane in each of twoorthogonal directions, the poles of said permanent magnet pieces beingaligned in like directions in said plane.
 2. An inductor as set forth inclaim 1, wherein said plurality of permanent magnet pieces are insulatedone from the other with insulating resin.
 3. An inductor as set forth inclaim 1, wherein said plurality of permanent magnet pieces are bondedwith an adhesive to the end faces defining said gap in said magneticcircuit.
 4. An inductor as set forth in claim 1, wherein said pluralityof permanent magnet pieces are bonded to a single sheet of anelectrically insulating material and then fixed in said gap defined insaid magnetic circuit.
 5. An inductor as set forth in claim 1, whereinsaid plurality of permanent magnet pieces are placed in a plurality ofrecessed portions provided in the end face of an iron core, said endface defining said gap, and the depth of said respective recessedportions being less in dimension than the thickness of said permanentmagnet.
 6. An inductor as set forth in claim 1, wherein the gap lengthof at least part of said gap defined in said magnetic circuit is less indimension than the thickness of said permanent magnet piece.
 7. Aninductor according to claim 1 wherein said plurality of permanent magnetpieces are selected from the group consisting of platinum-cobalt magnetsand rare earth-cobalt magnets.
 8. An inductor device comprising:amagnetic circuit made of soft magnetic material and having end facesdefining a gap in one position in said circuit; a coil wound around aportion of said magnetic circuit; and a permanent magnet plate disposedin said gap having a first surface defining a magnetic pole of a firstpolarity and a second surface defining a magnetic pole of a secondpolarity, a first set of substantially parallel grooves in said firstsurface and a second set of substantially parallel grooves in saidsecond surface, said first and second surfaces being substantiallyparallel with said end faces of said gap.
 9. The inductor according toclaim 8 wherein said first set of grooves is substantially parallel tosaid second set of grooves.
 10. The inductor according to claim 8wherein said first and second sets of grooves are offset in a directionorthogonal to said grooves.
 11. The inductor according to claim 8wherein said first and second sets of grooves are orthogonally disposedwith respect to each other.